The present invention is generally related to control techniques for performing power conversion in a multi-stage power converter system, and, more particularly, to method and system for performing DC/DC power conversion and controlling power flow between at least two separate power sources in a multi-stage power converter system.
Power converter systems, such as made up of a plurality of power converter stages connected in parallel and of essentially the same design, e.g., having substantially the same power ratings relative to one another, need to address various issues in order to provide cost-effective and reliable power conversion. For example, when the power converter system supplies power to a load, due to a variety of factors such as component variation, switching speed differences, offsets, etc., some of the converters could carry less of the load than others. Such an uneven load distribution among the converters is unacceptable. Further, the efficiency of the system is decreased because of uneven load distribution.
Another issue that may detrimentally impact efficient operation of power converter systems connected to separate power sources that, for example, are designed to operate at different voltage levels relative to one another, would arise if synchronous rectification techniques are not appropriately configured for those power conversion applications. Using synchronous rectification techniques, a switching device, such as a FET in parallel with its body diode is turned on during a freewheeling period to reduce the voltage drop and improve conversion efficiency. Using standard synchronous rectification techniques in a power converter connected to separate power sources can lead to a momentary but undesirable discharge of the power sources, if the switching device is not turned off in the event of reversal of current flow.
Another issue that is important in the operation of a power converter system, such as one connected to separate power sources, deals with reliably and economically protecting the system and associated equipment from fault conditions that may arise, such as in the power converter, in the power sources, in the loads or both. For example, when two or more power sources are connected through a DC/DC converter without galvanic isolation a means for quickly and automatically isolating the power sources needs to be provided to protect the sources from faults within the converter, or to protect the converter from external faults, for example, due to improper reversal of the connections for the sources.
Thus, in view of the foregoing discussion, it is desirable to provide system and control techniques that allow for improved forced current balance in parallel converters. It would also be desirable to provide system and control techniques that allow for improved synchronous rectification in the power switching devices. It would be further desirable to provide an improved input to output isolation and converter protection circuit.
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof a method for bi-directionally controlling power flow between at least two separate direct current power sources and loads in a multi-stage power converter system. The method allows providing at least first and second power converter stages coupled in parallel circuit to one another. The method further allows providing a first switching controller coupled to selectively actuate a pair of switching devices in the first power converter stage, and a second switching controller coupled to selectively actuate a pair of switching devices in the second power converter stage. One of the first and second switching controllers is selected as a master controller, and the other one of the first and second switching controllers is selected as a slave controller. An average voltage measurement is determined at a respective node of each of the power converter stages. A compensating signal is generated based on the difference of the average voltage measurements at the respective nodes of each of the power converter stages. The compensating signal is applied to the slave switching controller so that the pair of power switching devices coupled to that slave switching controller is selectively actuated to have a respective on and off duty cycle. The duty cycle imparted by the slave controller is adjustable in response to the compensating signal relative to the duty cycle imparted by the master controller to the pair of power switching devices coupled thereto in order to avoid current imbalance between the power converter stages. In other aspects of the invention, the method further allows improved techniques for performing synchronous rectification in the power switching devices, and providing input/output isolation and protection to the power system and/or power sources under faulty conditions.
The present invention further fulfills the foregoing needs by providing in another aspect thereof, a multi-stage power converter system for bi-directionally controlling power flow between at least two separate direct current power sources and loads. The power converter system includes at least first and second power converter stages coupled in parallel circuit to one another. The system further includes a first switching controller coupled to selectively actuate a pair of switching devices in the first power converter stage and a second switching controller coupled to selectively actuate a pair of switching devices in the second power converter stage. One of the first and second switching controllers is selected as a master controller, and the other one of the first and second switching controllers is selected as a slave controller. A feedback controller is configured to avoid current imbalance between the power converter stages by:
determining an average voltage measurement at a respective node of each the power converter stages;
generating a compensating signal based on the difference of the average voltage measurements at the respective nodes of each of the power converter stages; and
applying the compensating signal to the slave switching controller so that the pair of power switching devices coupled to that slave switching controller is selectively actuated to have a respective on and off duty cycle, the duty cycle imparted by the slave controller being adjustable in response to the compensating signal relative to the duty cycle imparted by the master controller to the pair of power switching devices coupled thereto, thus avoiding current imbalance between the power converter stages.